As a widely known type of rotating machine, there is an axial flow type turbine equipped with a casing, a rotating shaft that is rotatably provided inside the casing, a plurality of turbine vanes that are fixedly arranged at an inner peripheral portion of the casing, and a plurality of turbine blades that are radially provided on the rotating shaft on downstream sides of the plurality of turbine vanes. In the turbine, power is obtained by converting the pressure energy of a fluid into rotational energy. In addition, when the turbine is an impulse turbine, the pressure energy of the fluid is converted into kinematic energy by the turbine vanes, and this kinematic energy is converted into rotational energy by the turbine blades. Additionally, when the turbine is a reaction turbine, the pressure energy is converted into kinematic energy even within the turbine blades, and the kinematic energy is converted into rotational energy by a reaction force with which the fluid is jetted.
In such a turbine, gaps are formed in a radial direction between tip portions of the turbine blades that are rotary bodies (rotors), and a casing that is a stationary body (stator). Additionally, gaps in the radial direction are also formed between the tip portions of the turbine vanes that are stationary bodies and the rotating shaft that is a rotary body. For this reason, although a portion of the fluid leaks toward a downstream side through the gaps between the tip portions of the turbine blades and the casing, this leakage fluid does not apply rotary power to the turbine blades. Additionally, since the pressure energy of a leakage fluid, which passes toward the downstream side through the gaps between the tip portions of the turbine vanes and the rotating shaft, is not converted into kinematic energy by the turbine vanes, rotary power is hardly applied to the turbine blades on the downstream side. Therefore, in order to improve the performance of the turbine, it is important to reduce a fluid flow (leakage flow rate) that passes through the above gaps.
Here, for example, Patent Document 1 suggests a turbine having a structure in which a tip portion of a turbine blade is provided with a plurality of stepped portions the height of which becomes greater gradually from an upstream side toward a downstream side in an axial direction, the casings are provided with a plurality of seal fins that extend toward the respective stepped portions, and minute gaps are formed between the respective stepped portions and tips of the respective seal fins.
In this turbine, as a fluid that has entered the above gap from an upstream side collides against the stepped surface of each stepped portion, a main vortex is generated on the upstream side of the stepped surface. Additionally, a separation vortex resulting from the main vortex is generated on the downstream side (in the vicinity of the upstream side of the minute gap) of the stepped surface. By virtue of this separation vortex, the leakage flow rate is reduced by reducing a leakage flow that passes through the minute gap.